Method for manufacturing rolled steel products

ABSTRACT

A method for manufacturing rolled steel products, especially of threaded tension steels or the like, is described, in accordance with which the steel with a C-content of 0.50 to 0.80 W %, a Si-content of 0.20 to 0.60 W %, and a Mn-content of 0.30 to 0.80 W %, after hot rolling is surface quenched by means of cooling from the rolling heat at the output side of the finishing equipment so that the material in a peripheral zone is immediately and entirely converted to martensite, while the heat which remains in the core zone during the subsequent cooling effects an annealing the martensitic peripheral zone not beyond the range of the intermediate stage, and which in accordance with the invention is characterized in that after the cooling a cold working follows, and then an annealing. The total reduction amounts preferably to 0.5 to 1.5%; the annealing is preferably carried out at a temperature in the range of 350° C. to 380° C. and during a period of maximum temperature of 5 to 60 seconds. By means of this method rolled steel products, such as for example tension steels, can be made in a simple and economical way, which products have ductility and mechanical properties entirely meeting the demands required for construction.

The invention concerns a process for manufacturing rolled steelproducts, especially structural steels such as, for example, tensionsteels.

With respect to construction steels (heat treated steels) such as, forexample, tension steels, high demands are placed with regard tomechanical properties. Tension steels are used in building constructionas tensioning members for pre-stressed concrete, as anchor steels forearth and rock anchors, as concrete form anchors, for suspension cablesfor suspension bridges, cable ropes for cable rope bridges, staying andthe like. For a portion of these applications tension steels, preferablywith rod shaped across section are used in the strength class with yieldpoints between 800 and 1000 N/mm² (0,2-limit) and tensile strengthsbetween 1100 and 1300 N/mm². As to measurements, diameters from 12 to 50mm, especially from 20 to 40 mm, come into question. This involvestension steels with smooth outer surfaces or steels with for examplescrew shaped inclined ribs. Other possible realizations for tensionsteels are wires and sheet steel.

Tension steels along with static strength must also possess a very highelastic limit and a good ductility. In the case of threaded tensionsteels, that is such to which a threaded anchor can be applied, a highwear resistance for the outer surface as well as resistance to corrosionis also important Further important requirements are good relaxationproperties as well as an adequately high fatigue loading resistance. Rodshaped tension steels with diameters between about 12 and 50 mm are hotrolled to increase the yield point, subsequently stretched and thenannealed to remove stress. According to these methods indeed the lesserrequirements of the prescribed standards can be met, but the process isalso with respect to the steel composition (typical analysis in W%(Weight Percentage): C 0.75, Si 0.80, Mn 1.50, P 0.020, S0.020, V 0.25)and the carrying out of the method quite expensive and associated withhigh production costs. Beside numerous production steps, residualhydrogen and metallurgical segregation represent for this steel a bigproblem. The disadvantageous effect of cracks appearing during thestretching, as also of cracks which occur later, and the generalsusceptibility to corrosion of such tension steel are known. Theproduction disturbing high amount of rejects (scrap) in the plantrepresents another substantial cost factor.

A method for manufacturing rolled steel products, especially threadedtension steels and the like, is known from DE-OS 34 31 008, in which thesteel with a C-content of from 0.50 to 0.80 W%, a Si-content of from0.20 to 0.60 W%, and a Mn-content of from 0.30 to 0.80 W%, after hotrolling is cooled at the output side of the finishing equipment from therolling heat, especially by means of water (in principle cooling gascomes into question), so that a quenching of the outer surface takesplace such that the material in a peripheral zone is converted directlyand entirely into martensite, while the heat which remains in the corezone during the subsequent cooling effects an annealing of themartensitic peripheral zone not beyond the range of the intermediatestage.

In accordance with this method it is possible to use startingcompositions readily represented metallurgically and of cost effectiveanalysis for the manufacture of a tension steel which is resistant tocorrosion and having a wear resistant outer surface, which reduces thedanger of mechanical damage and suits the steel to the application ofthreads. The tension steel obtained through this method exhibitsmoreover, along with a high yield point and a high strength, a highductility or toughness above all at high temperatures, and it possessesalong with low relaxation a high fatigue strength.

From LU-A-65 413 a method for manufacturing a non-heat treated steel isknown, in which after the cooling a cold working is carried out with afollowing annealing. Since in the case of such steel, no metastablephase in the form of martensite appears which upon cold working caneffect a breaking of the material, a transfer of these measures to amethod according to DE-OS 34 31 008 cannot be considered obvious.

The object of the present invention is to provide an economical methodfor manufacturing rolled steel products, especially tension steels andwhich makes it possible to start from a simple and cost effectiveanalysis and to obtain from it in a simple and easily carried out way aproduct with properties which correspond highly and reliably with thoserequired for construction steels, especially tension steels. Thisproblem is solved by the present invention.

The subject of the invention is a method for manufacturing rolled steelproducts, especially threaded tension steels or the like, from steelswith a C-content of from 0.50 to 0.80 W%, Si-content of from 0.20 to0.60 W%, and Mn-content of from 0.30 to 0.80 W%, wherein after hotrolling, at the output side of the finishing equipment the steel iscooled from the rolling heat especially by means of a cooling fluid, forexample water, to effect such a quenching of the outer surface that thematerial in a peripheral zone is converted directly and entirely tomartensite, while the heat remaining in the core zone during thesubsequent cooling effects an annealing of the martensitic peripheralzone not beyond the range of the intermediate stage, which method ischaracterized in that after the foregoing a cold working takes place andthen subsequently an annealing. Practical refinements of this method arethe subjects of dependent claims 2 to 11.

With the method of the invention, through the method steps of coldworking and then subsequent annealing a two layered steel obtained afterhot rolling and outer surface quenching with annealed martensitic outerlayer, end products are obtained which further have up to 0.8 W%chromium, up to 0.5, especially 0.4 W% copper, up to 0.15 W% vanadium,up to about 0.06 W% niobium, up to 0.03 W% phosphorus, up to 0.03 W%sulfur, traces of titanium and/or traces of boron and/or nickel in aquantity such that the sum of chromium and nickel carries a W% up to0.8, especially up to 0.4, wherein these components can appearindividually or in combination with one another.

The starting material can itself be manufactured in a customary way, forexample in a block, or also as a continuous casting. A special treatmentfor removing hydrogen in either the liquid or the solid phase is alsousually necessary.

The semi-finished material is rolled for example on a thin materialrolling mill or a wire rolling mill to the end cross section. This hotrolling and the thereafter following controlled heat treatment (outersurface quenching) takes place preferably according to the methodversions and conditions described in DE-OS 34 31 008.

The end rolling temperature of the finishing equipment is preferably sochosen that it lies closely above A₃ at the lower limit of the hotforming range. The end rolling temperature lies preferably between 860°and 1060° C., and especially between 950° and 1000° C. The annealingduring the subsequent cooling takes place preferably such that the outersurface temperature of the peripheral zone in the time between thesecond and sixth seconds of the heat treatment, depending on the roddiameter, amounts to no more than about 500° C., and preferably amountsto between 400° and 500° C.

After the hot rolling and the controlled heat treatment as a result ofwhich a yield point value of about 900 N/mm² has been reached, a coldforming then follows. As cold forming twisting comes for example intoquestion. Preferred however is stretching since the resultingdeformation is largely homogeneous over the cross section. Thestretching is preferably done to a degree that corresponds in thestretch-strain line of the starting material somewhat to the region of(1.01 to 1.2)×Re, and especially (1.05 to 1.1)×Re, The total reductionamounts therefore preferably to 0.3 to 2.0%, and especially to 0.5 to1.5%.

The stretching can take place in the handling of the steel in a knownway and manner. Rods with a diameter of more than 15 mm (d_(s) greateror equal to 15 mm) are preferably individually stretched, and in thecase of wires a continuous stretching process can be undertaken as forexample is usual in the case of concrete steels. After stretching therefollows the annealing step of the invention to stabilize the defects anddisplacements obtained with the cold working. This annealing takes placeat a temperature in the range of from 300° to 420° C., especially from330° to 320° C., and most preferably in the range of from 350° to 380°C. The duration of maximum temperature amounts to preferably 5 to 60seconds, and especially about 10 seconds. This annealing can be carriedout in a usual way, for example in thermally heated ovens, orelectrically with conductive delivery of current, and preferably theheating takes place inductively, since in this case especially shortdurations are possible.

With the method of the invention it is possible to manufactureconstruction steel, especially tension steel, in a simple and economicalway, with the steel having a very high Re/Rm ratio; and with whichmethod for example the time consuming and risky method step of effusiontreatment is not necessary. The products manufactured according to themethod of the invention are suited on the basis of their properties verywell to the intended application; they can for their application have ausual form and can for example be formed as steel rods or wires withsmooth outer surfaces, or with suitable threads, ribs and the like, asdescribed for example in DE-OS 34 31 008. The products have sufficientductility, a high R_(P) 0.01 value (technical elasticity limit), a smallrelaxation and a sufficient elongation. As a comparison of example 1(the method according to DE-OS 34 31 008) and example 2 (the method ofthe invention) shows, the products made according to the method of thepresent invention exhibit for similar breaking elongations, bettervalues for the yield point (R_(e)), and the tensile strength (R_(m)) andthe relaxation (T).

The following examples are intended to further explain the inventionwithout limiting it.

EXAMPLES EXAMPLE 1 (COMPARISON EXAMPLE)

A steel with the composition (in W%): C 0.68; Si 0.35; Mn 0.66; P 0.021and S 0.025 was rolled as ribbed steel (threaded steel) and wassubjected to the heat treatment method of DE-OS 34 31 008. The followingvalues were obtained:

Yield Point (R_(e)): 900 N/mm²

Tensile Strength (R_(m)): 1200 N/mm²

Breaking Elongation (A₁₀): 10.3%

Relaxation (T₁₀₀₀): 4 to 6% (1000 hours; i=0.8×Rm)

EXAMPLE 2 (METHOD OF THE INVENTION)

The product obtained from the method of example 1 was thereafterstretched 0.7% after which an annealing took place at 350° C. (10seconds). The following values were obtained:

Yield Point (R_(e)): 1100 N/mm²

Tensile Strength (R_(m)): 1250 N/mm²

Breaking Elongation (A₁₀): 9.8%

Relaxation (T₁₀₀₀): <2% (1000 hours; _(i) =0.8×Rm)

Bending Capacity: 5×d_(s) (to 180°, no break)

The product obtained according to example 2 had a corrosion resistanceof as good a value as the steel manufactured according to example 1.

The method of the invention especially distinguishes itself in that witha cost effectively obtainable starting material and with method stepssimple to execute (for example without separate tempering steps) aproduct with improved material properties, especially improved yieldpoint, tensile strength and relaxation, is obtained. It is also anadvantage that all of the products obtained in accordance with theinvention have been automatically tested as to their ability towithstand a tensile loading since these products have successfullyendured the stretching treatment.

We claim:
 1. A method for manufacturing rolled steel products with steelhaving a C-content of 0.50 to 0.80 Wt%, a Si-content of 0.20 to 0.60Wt%, and Mn-content of 0.30 to 0.80 Wt%, said method comprising thesteps of:hot rolling said steel; surface quenching said steel from therolling heat at the output side of the rolling equipment by means ofcooling so that a peripheral zone of said steel is converted tomartensite, subsequently further cooling said steel, during whichsubsequent cooling the heat remaining in the core zone of said steeleffects a partial annealing of the martensitic peripheral zone; thencold working said steel; and thereafter annealing said steel.
 2. Themethod of claim 1, wherein said cold working comprises stretching. 3.The method of claim 2, wherein said steel is reduced by 0.3 to 2.0%. 4.The method of claim 2, wherein said steel is reduced by 0.5 to 1.5%. 5.The method of claim 1, wherein said annealing step is carried out in atemperature range of 300° to 400° degrees C.
 6. The method of claim 1,wherein said annealing step is carried out in a temperature range of330° to 420° C.
 7. The method of claim 1, wherein said annealing step iscarried out in a temperature range of 350° to 380° C.
 8. The method ofclaim 1, wherein said annealing step is carried out during a maximumtemperature period of 5 to 60 seconds.
 9. The method of claim 1, whereinsaid annealing step is carried out during a maximum temperature periodof 10 seconds.
 10. The method of claim 8, wherein said annealing stepincludes induction heating of said steel.
 11. The method of claim 1,wherein said temperature of said steel at the output side of the rollingequipment lies at the lower limit of the heat formability of said steelbarely over A3.
 12. The method of claim 1, wherein said temperature ofsaid steel at the output side of the rolling equipment is between 860°and 1060° C.
 13. The method of claim 11, wherein said temperature ofsaid steel at the output side of the rolling equipment is between 950°and 1000° C.
 14. The method of claim 1, wherein during said subsequentcooling of said steel the outer surface temperature of said martensiticperipheral zone in the time period between the second and sixth secondsof the heat treatment does not exceed 500° C.
 15. The method of claim 1,wherein during said subsequent cooling of said steel the outer surfacetemperature of said martensitic peripheral zone in the time periodbetween the second and sixth seconds of the heat treatment is between400° and 500° C.
 16. The method of claim 1, wherein said steelcomprises: up to 0.8 Wt% chromium, up to 0.5 Wt% copper, up to 0.15 Wt%vanadium, up to 0.06 Wt% niobium, up to 0.03 Wt% phosphorus, up to 0.03Wt% sulfur, and possible traces of titanium, boron and nickel, and,wherein the sum of said chromium and said nickel amounts to up to 0.8Wt%.